Proceedings from Conference ’97: Searching for Quality. Joint Meeting of the Australian Avocado
Grower’s Federation, Inc. and NZ Avocado Growers Association, Inc., 23-26 September 1997. J. G.
Cutting (Ed.). Pages 147-154.
AVOCADO IRRIGATION PRACTICAL OBSERVATIONS IN
DETERMINING WATER NEED, IRRIGATION DESIGN AND
FREQUENCY SCHEDULING
C J Partridge
Avo Systems Limited, 109 Oliver Road, Tauranga, New Zealand
Summary
While irrigation is felt to be essential for successful avocado production in many
countries, it is not a widely adopted practice in New Zealand and very little or no data is
available to support the author's contention that lack of irrigation could be a limiting
factor. The aim of this paper is to hopefully stimulate some progressive growers to
experiment with the concept and at the same time, provide some guidelines as a
starting point for local conditions.
Introduction
There are some factors fundamental to the successful cultivation of avocados (Pegg et
al, 1987).
•
soil selection
•
using good quality plant material
•
irrigation
•
management
•
fertilisation
•
pest and disease control
Provided the prevailing general climatic conditions are conducive to fruit set, it is the
successful integration of the above factors which will encourage, maximise and sustain
avocado yield in any given orchard. For many years, Phytophthora rootrot was the
scourge of avocado orchards worldwide and was the most limiting factor for profitable
production. The advent of effective control measures (Darvas et al, 1987), is now
reason for every grower to re-evaluate the factors most limiting on his or her orchard
and it is the author's contention that, irrigation management or the lack thereof, could
prove to be the next most limiting factor for avocados in New Zealand.
While there is a fair amount of published research on avocado irrigation worldwide,
there is a distinct lack of information on irrigation under New Zealand conditions. In fact,
the majority of New Zealand growers do not have irrigation systems installed (Cutting,
pers com). By reviewing some research results of other countries and combining this
with the author's field experiences, primarily in South Africa, it is hoped that interest will
be stimulated amongst local growers and possibly even cause growers with irrigation
here and elsewhere, to revisit their systems. It is not within the scope of this paper to
extensively review equipment or irrigation engineering, as it is felt that armed with
relevant horticultural knowledge, irrigation engineers should be able to design to the
tree's requirements.
At this point it should be stressed that the avocado industry in New Zealand is based
almost exclusively on the variety 'Hass' and while research elsewhere has been done
on other varieties, the main focus of this paper will be on the requirements of the 'Hass'
variety.
The avocado tree's requirements
Avocados evolved under warm to cool subtropical rainforest conditions where rain is
evenly distributed throughout the year and where they grow on well drained, leached
soils. It is generally accepted that water is essential for growth, fruiting and producing
marketable sized fruit and that New Zealand is blessed with an abundance of the
precious commodity. What should be of interest to growers though, is the distribution of
that rain and whether there is sufficient, in normal years, to supply the tree's needs in
terms of photosynthetic efficiency, nutrient uptake etc. at the critical times.
Whiley et a/.(1988), used phenological growth cycles to identify critical periods for
various inputs such as irrigation and nutrition. This excellent work provided a platform
for better understanding of avocado tree management but is more directly applicable to
Australian and South African conditions where summer rainfall and warmer
temperatures are the norm and also, the work was done on the 'Fuerte' cultivar.
More recently, Thorpe et al. (1995) reported on tree growth cycles under New Zealand
conditions this important work should ideally be reconfirmed since the local grower
community has had time to implement better rootrot control practices. Also, rain falls in
New Zealand mainly in winter and growing conditions are generally cool. It can
therefore be expected that growth cycles will be peculiar to local conditions.
It is well known that the act of flowering and fruitset is in itself, a stressful time for the
tree. Any factor, which increases the stress load, will decrease the potential of the tree
to maximise its production. It has been estimated that a tree in full flower, increases its
surface area for potential water loss through soft flower tissue, by up to 80%. Apart from
the avocado's genetic limitations on fruitset, stress will aggravate the already low
propensity (as low as only 0.4% of total flowers setting) of the tree to set fruit
(Wolstenholme,1985).
Even when the tree manages to set a good crop, it still has to regenerate vegetative
growth and roots if it is to stand any chance of fruiting the following season and these
processes will be curtailed by any stress. Whiley (1994) in his doctoral thesis, while not
referring specifically to irrigation, confirms the necessity of leaf retention and root
regeneration for sustained cropping.
Wolstenholme (1989) makes reference to the energy expense to the tree of producing
an oil containing fruit. In New Zealand, many orchards will be carrying fruit from the
previous season while flowering and setting for the coming season in any given year, so
energy demands can be expected to be high under these conditions.
The growers' and the markets' requirements
Simply put, growers require a sustainable return on their long term investment and have
to satisfy the demands of the markets in order to achieve this. The avocado tree's habit
of alternate bearing can frustrate both the grower and the market.
Minimising tree stress to try and even out the bearing cycles is one avenue for the
grower to follow, but the picture is further complicated by the fact that fruit volume alone
will not satisfy the market. Speak to any fruit marketer, and the message is clear: the
market increasingly demands the larger sized fruit with good internal and external
quality.
Careful irrigation scheduling and management, coupled with good nutrition, can improve
fruit size in Hass dramatically. Under South African conditions, small Hass fruit size is
an ongoing problem (Moore-Gordon et al, 1996). It was interesting to the author in the
mid 1980's, to observe a situation in Kiepersol in The Eastern Transvaal where a grower
interplanted Hass in a banana plantation which was due to be removed over time. The
grower in question continued to apply a banana irrigation and fertiliser regime, which
was lavish by previous conventional avocado wisdom, resulting in the production of over
15 tons per ha of the largest Hass fruit seen until that time. This sowed the seed that the
conventional wisdom based mainly on Fuerte work, was not ideal for Hass and
prompted the author to review his advice to growers.
Since then various researchers have confirmed the positive effect of good irrigation
practises on fruit size and yield in Hass and van Eyk (1994) reported dramatic yield
improvements over two years following a heavy irrigation regime (between field capacity
and -40kpa tensiometer reading) with Hass on clonal Duke 7 rootstock.
Moore-Gordon et al. (1996) working on a mulching trial, reported an increase of 11.88%
in mean fruit mass and 16.7% more fruit over two seasons, when mulching was applied.
This was directly attributable to a reduction of plant water stress.
Bower (1984 & 1986) working with Fuerte, reported improved post harvest quality and
implicated pre-harvest water stress in predisposing fruit to post harvest quality
problems. It is extremely likely that the same principles will apply to Hass.
How much water is required to irrigate avocados?
Soil water holding capacity and prevailing weather conditions will play a major role in
water usage, but provided some method of measuring soil moisture is employed, a
grower can at least determine when water is needed.
However, it is all very well to have a guideline of irrigate at a tensiometer reading of -40
kpa' for example, but this does not give any idea of how much stored or available water
is required over a period of time. In order to try and make a 'guestimate' of what is
necessary under New Zealand conditions, it is useful to review some case studies:
•
In South Africa, it was long suspected that Hass had different water requirements to
Fuerte, the dominant variety. The first inkling that this was true, came in the mid
1980's when a grower in the Levubu area established a block of Hass and Fuerte,
both on the same rootstock, at the same time on identical soils and separated only
by a road. Using tensiometers, he noticed that those in the Hass block reached the
irrigation reading twice as fast as the Fuerte. Unfortunately his actual application
rates were never recorded but as a rule of thumb, it is now accepted that Hass
requires twice as much water as Fuerte
•
Grower A tried to irrigate with microjets according to tensiometers during a very dry
period on a 10 year old Hass block on Edranol seedling rootstock. Because bananas
also had a priority, he was unable to irrigate optimally when the tensiometers
indicated that the soil was dry and therefore irrigated when he could on a bi-weekly
basis for 12 hours at a time, from June 1993 to December 1993. During this time no
rain fell and the trees went through flowering and fruitset (critical times for water
needs). Actual water usage was measured and it was found to be 611712 liters per
month (or an average of 153000 liters per week). This resulted in a crop of just
under 20 tons per hectare, but more importantly, the grower said he would have
irrigated more heavily and more often if he could have
•
Grower B has a well managed, mature block (tree canopies nearly covering the total
planted surface area) of Hass on clonal Duke 7 irrigated with microjets.
Tensiometers were never installed despite much encouragement! Irrigation was
applied weekly for a set time of 8 hours at a time and other management inputs were
carried out diligently. The orchard had been averaging over 15 tons per ha over four
years and water usage was calculated to be in the order of 240 000 liters per ha per
month during the critical flowering and fruitset periods. The yields achieved were far
above average but in the author's opinion, could have been even better if irrigation
was more scientifically scheduled
•
Staff at Westfalia Estate in South Africa, have estimated that they try and apply the
equivalent of 30-35 mm rainfall per week to the mature producing Hass orchards
during flowering and fruit set. If it is assumed that about 65% of the surface area is
wetted by their microjet irrigation system, this equates to around 240 000 liters per
hectare per week which is similar to the case study above
•
Lahav et al (1991), reports that 12700 cubic meters of water per ha per year was the
minimum required for avocado orchards in Western Galilee, Israel, for desired
cropping using under-canopy sprinklers. The introduction of drip and microjet
irrigation allowed reductions in water use, but that within the parameters of the trial,
every reduction in water applied, resulted in a corresponding yield reduction. Salinity
is an additional complicating factor in Israel
Accepting that weather conditions are generally much less harsh in New Zealand over
the dry summer months compared to those in South Africa or Israel, plus the fact the
soils are generally well drained, a guestimate of water requirements in New Zealand
would be: 120-150000 liters per hectare per week on mature orchards, during the
critical periods. In making this guess, only careful monitoring of actual requirements and
applications over time will prove it right or wrong.
Obviously water requirements will be much less in the early years of an orchard's life.
However, most avocados are grown in regions of the world where rain does fall heavily
at some time of the year. The trees naturally grow out to colonise new areas during
these times and so the rooted area expands when conditions permit. It is therefore very
important to design for the mature orchard's requirements. By 'maturity', it is meant that
stage when the total planted area is almost fully covered with leaf canopy; this is a
function of planting distance and not necessarily tree age. Also, an irrigation system
should ideally be designed to cover 65-70% of the canopy surface area with water at
orchard maturity so that irrigated water adequately covers the rooted zone.
Maximum water requirements during critical periods, implies that this demand will only
occur for a relatively short period of the growing season as cooler weather or rainfall will
reduce irrigation demand at other times; in effect, a case of not always needing the
capacity, but needing it badly during key events in the trees production cycle. In New
Zealand, this will probably be at times from late September to February, corresponding
to the flowering and fruit set period.
Irrigation systems
Apart from gentle rain, there is no perfect irrigation system although designers are
getting close to it. Some, like high volume, dragline or floppy sprinkler systems used in
South Africa are very good from a water distribution point of view, but wasteful in the
early years and can be labour intensive to operate. Others, like drip irrigation are
suitable initially but end up not being able to distribute the water adequately and are
prone to blockages; in the author's opinion drippers are not recommended for avocados
at all. Micro-jet irrigation systems provide a compromise, can be expensive, but together
with the floppy sprinkler system would appear to be most suited to local conditions.
It is felt that the ideal system would have the following characteristics:
•
Capable of delivering water to a limited area under each tree when young to reduce
water wastage initially
•
Capable of increasing the wetted area to take into account tree growth and cover 6070% of the canopy area at orchard maturity. In the design of the system, sufficient
pressure and pipe delivery must be designed for, as the system has to expand
•
Not be prone to blockages
•
Should be capable of delivering 60 liters of water per hour per emitter or more ie. be
capable of high delivery rates
•
Should be designed such that the cycle time between irrigations does not have to
exceed one week. On well-drained soils, stress conditions can develop rapidly
especially during periods of hot weather
•
Be designed such that different cultivars, age groups, or blocks on different soil
types, are capable of being irrigated as separate entities. This is important as water
requirements can differ
Managing and scheduling irrigation
An orchard's water requirements can vary considerably over a short period of time.
Calendar irrigation scheduling can not take into account the often subtle changes in soil
moisture status and additionally, the avocado does not thrive under poorly aerated soil
conditions (Wolstenholme, undated), so a drying cycle is required; this can only be
achieved by using some form of soil moisture measuring device.
There are some sophisticated scheduling devices on the market, unfortunately often at
prices too sophisticated for the average grower and the following are some examples of
what is available:
•
Evaporation pan
•
Tensiometers
•
Neutron probes
•
Enviroscan
While it is not the intention to provide a list of pros and cons of each device it is this
authors assumption, that tensiometers would be the choice of the average grower. This
is because despite some shortcomings, they have proved their reliability in the field,
have been tested under experimental conditions, are relatively inexpensive and are
easy to install and maintain. Their drawbacks should be mentioned as well however and
these are:
•
They measure moisture in a very limited soil area and so careful site selection is
necessary
•
They require discipline on behalf of the grower to read and service regularly
At a minimum, two tensiometers should be installed per irrigation block; a 30cm and a
60cm tensiometer with the shallow to indicate cycle frequency and the deep one cycle
duration. It is also very important that they be installed within the area wetted by the
irrigation system, of a representative tree in the block, on the dripline of the warmest
side of the tree.
The pro and cons of mulching are reviewed in an article by Wolstenholme et al (1996). It
would appear however, that researchers and orchard managers are overlooking a
possibility of fine tuning irrigation needs even further: avocado roots thrive in well
aerated soil conditions and mulching encourages root development in the mulch and at
the soil/mulch interface. Thereafter these roots are often ignored from an irrigation point
of view. This author would argue that there would be benefit in installing 15cm
tensiometers wherever mulching has been applied to monitor the moisture conditions in
this zone, where some of the healthiest roots will be found.
In the light of research results and personal observations, it is felt that if using
tensiometers, irrigation should commence at a reading of-40kpa on clays and volcanic
soils in New Zealand and -30kpa on sandy soils. Although a slight stress period may be
thought to be beneficial under some circumstances in other countries, the fact that two
crops are generally hanging on the trees at any one time, is probably stressful enough.
Irrigation and fertilisation
Fertiliser cannot be effectively utilised by the tree in the absence of available soil
moisture. Having an irrigation system therefore allows the grower to be more certain
that the fertiliser inputs are effective at the desired times. This is of especial note under
New Zealand conditions where a lot of essential fertiliser is required during the summer
growing period when rainfall is erratic.
Over-irrigation
Avocado roots are intolerant of prolonged periods of poorly aerated or waterlogged
conditions. Additionally, Phytophthora rootrot is encouraged by such conditions. It has
been observed that young trees especially, can be over-irrigated while their roots are
confined in a fairly small soil area and so, growers should take care to manage irrigation
especially carefully during the first year of establishment.
Where to from here for New Zealand avocado growers?
If you already have irrigation installed, reading this paper will hopefully give you some
useful information on how to re-examine your system and its management.
If you are unsure of whether irrigation is required or not, before committing a substantial
capital outlay to an irrigation system, you could purchase some tensiometers to monitor
the soil moisture conditions in your orchard over a period of a full season. Whatever the
results, they will be interesting and will be a very useful aid to determine whether you
should be considering installing an irrigation system.
References
Bower, J. P. 1984. Effect of fruit water stress and irrigation regime in the ripening of
stored avocado fruit, cultivar; Fuerte. South African Avocado Growers Association
Yearbook 7, 55-56.
Bower, J. P. 1986. Long-term irrigation as influencing avocado abscisic acid content
and fruit quality. South African Avocado Growers Association Yearbook 9, 43-45.
Cutting, J.G.M. 1997. Personal communication. Darvas, J.M. & Bezuidenhout, JJ. 1987.
Control of Phytophthora root rot of avocados by trunk injection. Proceedings of the
First World Avocado Congress, 91-93.
Lahav, E., Steinhardt, R. & Raimar, D. 1992. Proceedings of the Second World
Avocado Congress 1992, 323-330.
Moore-Gordon, C. S. & Wolstenholme, B. N. 1996. The Hass small-fruit problem: role of
physiological stress and its amelioration by mulching. South African Avocado
Growers Association Yearbook 19, 82-86.
Pegg, K.G. & Whiley, A.W. 1987. Phytophthora control in Australia. Proceedings of the
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Thorpe, G., Anderson & Camilleri. 1995. A New Zealand model of avocado tree growth
cycles. A report to the New Zealand Avocado Growers Association,
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envruggrootte in Hass avokado. Voorlopige resultate. South African Avocado
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Whiley, A. W., Saranah, J.B., Cull, B.W. & Pegg, K.G. 1988a. Manage avocado tree
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yield potential in avocado (Persea americana Mill.). Doctoral Thesis, Department of
Horticultural Science, University of Natal, Pietermaritzburg.
Wolstenholme, B.N. undated. Soil Requirements for avocado growing. Leaflet of the
University of Natal, Pietermaritzburg, South Africa.
Wolstenholme, B.N., Moore-Gordon, C. & Ansermino, S.D. 1996. Some pros and cons
of Mulching Avocado Orchards. South African Avocado Growers Association
Yearbook 19, 87-91.
Wolstenholme, B.N. 1985. Why do avocado fruits drop prematurely? AVOKAD; Journal
of the South African Avocado Growers Association 5.2, 3.
Wolstenholme, B.N. 1989. Carbohydrate management and avocado productivity. A
VOKAD; Journal of the South African Avocado Growers Association 9.1, 4.